// This file is part of Substrate.

// Copyright (C) 2019-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! A slashing implementation for NPoS systems.
//!
//! For the purposes of the economic model, it is easiest to think of each validator as a nominator
//! which nominates only its own identity.
//!
//! The act of nomination signals intent to unify economic identity with the validator - to take
//! part in the rewards of a job well done, and to take part in the punishment of a job done badly.
//!
//! There are 3 main difficulties to account for with slashing in NPoS:
//!   - A nominator can nominate multiple validators and be slashed via any of them.
//!   - Until slashed, stake is reused from era to era. Nominating with N coins for E eras in a row
//!     does not mean you have N*E coins to be slashed - you've only ever had N.
//!   - Slashable offences can be found after the fact and out of order.
//!
//! The algorithm implemented in this module tries to balance these 3 difficulties.
//!
//! First, we only slash participants for the _maximum_ slash they receive in some time period,
//! rather than the sum. This ensures a protection from overslashing.
//!
//! Second, we do not want the time period (or "span") that the maximum is computed
//! over to last indefinitely. That would allow participants to begin acting with
//! impunity after some point, fearing no further repercussions. For that reason, we
//! automatically "chill" validators and withdraw a nominator's nomination after a slashing event,
//! requiring them to re-enlist voluntarily (acknowledging the slash) and begin a new
//! slashing span.
//!
//! Typically, you will have a single slashing event per slashing span. Only in the case
//! where a validator releases many misbehaviors at once, or goes "back in time" to misbehave in
//! eras that have already passed, would you encounter situations where a slashing span
//! has multiple misbehaviors. However, accounting for such cases is necessary
//! to deter a class of "rage-quit" attacks.
//!
//! Based on research at <https://research.web3.foundation/en/latest/polkadot/slashing/npos/>

use super::{
    BalanceOf, Config, EraIndex, Error, Exposure, Module, NegativeImbalanceOf, Perbill, SessionInterface, Store,
    UnappliedSlash,
};
use codec::{Decode, Encode};
use frame_support::{
    ensure,
    traits::{Currency, Imbalance, OnUnbalanced},
    StorageDoubleMap, StorageMap,
};
use sp_runtime::{
    traits::{Saturating, Zero},
    DispatchResult, RuntimeDebug,
};
use sp_std::vec::Vec;

/// The proportion of the slashing reward to be paid out on the first slashing detection.
/// This is f_1 in the paper.
const REWARD_F1: Perbill = Perbill::from_percent(50);

/// The index of a slashing span - unique to each stash.
pub type SpanIndex = u32;

// A range of start..end eras for a slashing span.
#[derive(Encode, Decode)]
#[cfg_attr(test, derive(Debug, PartialEq))]
pub(crate) struct SlashingSpan {
    pub(crate) index: SpanIndex,
    pub(crate) start: EraIndex,
    pub(crate) length: Option<EraIndex>, // the ongoing slashing span has indeterminate length.
}

impl SlashingSpan {
    fn contains_era(&self, era: EraIndex) -> bool {
        self.start <= era && self.length.map_or(true, |l| self.start + l > era)
    }
}

/// An encoding of all of a nominator's slashing spans.
#[derive(Encode, Decode, RuntimeDebug)]
pub struct SlashingSpans {
    // the index of the current slashing span of the nominator. different for
    // every stash, resets when the account hits free balance 0.
    span_index: SpanIndex,
    // the start era of the most recent (ongoing) slashing span.
    last_start: EraIndex,
    // the last era at which a non-zero slash occurred.
    last_nonzero_slash: EraIndex,
    // all prior slashing spans' start indices, in reverse order (most recent first)
    // encoded as offsets relative to the slashing span after it.
    prior: Vec<EraIndex>,
}

impl SlashingSpans {
    // creates a new record of slashing spans for a stash, starting at the beginning
    // of the bonding period, relative to now.
    pub(crate) fn new(window_start: EraIndex) -> Self {
        SlashingSpans {
            span_index: 0,
            last_start: window_start,
            // initialize to zero, as this structure is lazily created until
            // the first slash is applied. setting equal to `window_start` would
            // put a time limit on nominations.
            last_nonzero_slash: 0,
            prior: Vec::new(),
        }
    }

    // update the slashing spans to reflect the start of a new span at the era after `now`
    // returns `true` if a new span was started, `false` otherwise. `false` indicates
    // that internal state is unchanged.
    pub(crate) fn end_span(&mut self, now: EraIndex) -> bool {
        let next_start = now + 1;
        if next_start <= self.last_start {
            return false;
        }

        let last_length = next_start - self.last_start;
        self.prior.insert(0, last_length);
        self.last_start = next_start;
        self.span_index += 1;
        true
    }

    // an iterator over all slashing spans in _reverse_ order - most recent first.
    pub(crate) fn iter(&'_ self) -> impl Iterator<Item = SlashingSpan> + '_ {
        let mut last_start = self.last_start;
        let mut index = self.span_index;
        let last = SlashingSpan { index, start: last_start, length: None };
        let prior = self.prior.iter().cloned().map(move |length| {
            let start = last_start - length;
            last_start = start;
            index -= 1;

            SlashingSpan { index, start, length: Some(length) }
        });

        sp_std::iter::once(last).chain(prior)
    }

    /// Yields the era index where the most recent non-zero slash occurred.
    pub fn last_nonzero_slash(&self) -> EraIndex {
        self.last_nonzero_slash
    }

    // prune the slashing spans against a window, whose start era index is given.
    //
    // If this returns `Some`, then it includes a range start..end of all the span
    // indices which were pruned.
    fn prune(&mut self, window_start: EraIndex) -> Option<(SpanIndex, SpanIndex)> {
        let old_idx = self
            .iter()
            .skip(1) // skip ongoing span.
            .position(|span| span.length.map_or(false, |len| span.start + len <= window_start));

        let earliest_span_index = self.span_index - self.prior.len() as SpanIndex;
        let pruned = match old_idx {
            Some(o) => {
                self.prior.truncate(o);
                let new_earliest = self.span_index - self.prior.len() as SpanIndex;
                Some((earliest_span_index, new_earliest))
            },
            None => None,
        };

        // readjust the ongoing span, if it started before the beginning of the window.
        self.last_start = sp_std::cmp::max(self.last_start, window_start);
        pruned
    }
}

/// A slashing-span record for a particular stash.
#[derive(Encode, Decode, Default)]
pub(crate) struct SpanRecord<Balance> {
    slashed: Balance,
    paid_out: Balance,
}

impl<Balance> SpanRecord<Balance> {
    /// The value of stash balance slashed in this span.
    #[cfg(test)]
    pub(crate) fn amount_slashed(&self) -> &Balance {
        &self.slashed
    }
}

/// Parameters for performing a slash.
#[derive(Clone)]
pub(crate) struct SlashParams<'a, T: 'a + Config> {
    /// The stash account being slashed.
    pub(crate) stash: &'a T::AccountId,
    /// The proportion of the slash.
    pub(crate) slash: Perbill,
    /// The exposure of the stash and all nominators.
    pub(crate) exposure: &'a Exposure<T::AccountId, BalanceOf<T>>,
    /// The era where the offence occurred.
    pub(crate) slash_era: EraIndex,
    /// The first era in the current bonding period.
    pub(crate) window_start: EraIndex,
    /// The current era.
    pub(crate) now: EraIndex,
    /// The maximum percentage of a slash that ever gets paid out.
    /// This is f_inf in the paper.
    pub(crate) reward_proportion: Perbill,
}

/// Computes a slash of a validator and nominators. It returns an unapplied
/// record to be applied at some later point. Slashing metadata is updated in storage,
/// since unapplied records are only rarely intended to be dropped.
///
/// The pending slash record returned does not have initialized reporters. Those have
/// to be set at a higher level, if any.
pub(crate) fn compute_slash<T: Config>(params: SlashParams<T>) -> Option<UnappliedSlash<T::AccountId, BalanceOf<T>>> {
    let SlashParams { stash, slash, exposure, slash_era, window_start, now, reward_proportion } = params.clone();

    let mut reward_payout = Zero::zero();
    let mut val_slashed = Zero::zero();

    // is the slash amount here a maximum for the era?
    let own_slash = slash * exposure.own;
    if slash * exposure.total == Zero::zero() {
        // kick out the validator even if they won't be slashed,
        // as long as the misbehavior is from their most recent slashing span.
        kick_out_if_recent::<T>(params);
        return None;
    }

    let (prior_slash_p, _era_slash) =
        <Module<T> as Store>::ValidatorSlashInEra::get(&slash_era, stash).unwrap_or((Perbill::zero(), Zero::zero()));

    // compare slash proportions rather than slash values to avoid issues due to rounding
    // error.
    if slash.deconstruct() > prior_slash_p.deconstruct() {
        <Module<T> as Store>::ValidatorSlashInEra::insert(&slash_era, stash, &(slash, own_slash));
    } else {
        // we slash based on the max in era - this new event is not the max,
        // so neither the validator or any nominators will need an update.
        //
        // this does lead to a divergence of our system from the paper, which
        // pays out some reward even if the latest report is not max-in-era.
        // we opt to avoid the nominator lookups and edits and leave more rewards
        // for more drastic misbehavior.
        return None;
    }

    // apply slash to validator.
    {
        let mut spans = fetch_spans::<T>(stash, window_start, &mut reward_payout, &mut val_slashed, reward_proportion);

        let target_span = spans.compare_and_update_span_slash(slash_era, own_slash);

        if target_span == Some(spans.span_index()) {
            // misbehavior occurred within the current slashing span - take appropriate
            // actions.

            // chill the validator - it misbehaved in the current span and should
            // not continue in the next election. also end the slashing span.
            spans.end_span(now);
            <Module<T>>::chill_stash(stash);

            // make sure to disable validator till the end of this session
            if T::SessionInterface::disable_validator(stash).unwrap_or(false) {
                // force a new era, to select a new validator set
                <Module<T>>::ensure_new_era()
            }
        }
    }

    let mut nominators_slashed = Vec::new();
    reward_payout += slash_nominators::<T>(params, prior_slash_p, &mut nominators_slashed);

    Some(UnappliedSlash {
        validator: stash.clone(),
        own: val_slashed,
        others: nominators_slashed,
        reporters: Vec::new(),
        payout: reward_payout,
    })
}

// doesn't apply any slash, but kicks out the validator if the misbehavior is from
// the most recent slashing span.
fn kick_out_if_recent<T: Config>(params: SlashParams<T>) {
    // these are not updated by era-span or end-span.
    let mut reward_payout = Zero::zero();
    let mut val_slashed = Zero::zero();
    let mut spans = fetch_spans::<T>(
        params.stash,
        params.window_start,
        &mut reward_payout,
        &mut val_slashed,
        params.reward_proportion,
    );

    if spans.era_span(params.slash_era).map(|s| s.index) == Some(spans.span_index()) {
        spans.end_span(params.now);
        <Module<T>>::chill_stash(params.stash);

        // make sure to disable validator till the end of this session
        if T::SessionInterface::disable_validator(params.stash).unwrap_or(false) {
            // force a new era, to select a new validator set
            <Module<T>>::ensure_new_era()
        }
    }
}

/// Slash nominators. Accepts general parameters and the prior slash percentage of the validator.
///
/// Returns the amount of reward to pay out.
fn slash_nominators<T: Config>(
    params: SlashParams<T>,
    prior_slash_p: Perbill,
    nominators_slashed: &mut Vec<(T::AccountId, BalanceOf<T>)>,
) -> BalanceOf<T> {
    let SlashParams { stash: _, slash, exposure, slash_era, window_start, now, reward_proportion } = params;

    let mut reward_payout = Zero::zero();

    nominators_slashed.reserve(exposure.others.len());
    for nominator in &exposure.others {
        let stash = &nominator.who;
        let mut nom_slashed = Zero::zero();

        // the era slash of a nominator always grows, if the validator
        // had a new max slash for the era.
        let era_slash = {
            let own_slash_prior = prior_slash_p * nominator.value;
            let own_slash_by_validator = slash * nominator.value;
            let own_slash_difference = own_slash_by_validator.saturating_sub(own_slash_prior);

            let mut era_slash =
                <Module<T> as Store>::NominatorSlashInEra::get(&slash_era, stash).unwrap_or_else(|| Zero::zero());

            era_slash += own_slash_difference;

            <Module<T> as Store>::NominatorSlashInEra::insert(&slash_era, stash, &era_slash);

            era_slash
        };

        // compare the era slash against other eras in the same span.
        {
            let mut spans =
                fetch_spans::<T>(stash, window_start, &mut reward_payout, &mut nom_slashed, reward_proportion);

            let target_span = spans.compare_and_update_span_slash(slash_era, era_slash);

            if target_span == Some(spans.span_index()) {
                // End the span, but don't chill the nominator. its nomination
                // on this validator will be ignored in the future.
                spans.end_span(now);
            }
        }

        nominators_slashed.push((stash.clone(), nom_slashed));
    }

    reward_payout
}

// helper struct for managing a set of spans we are currently inspecting.
// writes alterations to disk on drop, but only if a slash has been carried out.
//
// NOTE: alterations to slashing metadata should not be done after this is dropped.
// dropping this struct applies any necessary slashes, which can lead to free balance
// being 0, and the account being garbage-collected -- a dead account should get no new
// metadata.
struct InspectingSpans<'a, T: Config + 'a> {
    dirty: bool,
    window_start: EraIndex,
    stash: &'a T::AccountId,
    spans: SlashingSpans,
    paid_out: &'a mut BalanceOf<T>,
    slash_of: &'a mut BalanceOf<T>,
    reward_proportion: Perbill,
    _marker: sp_std::marker::PhantomData<T>,
}

// fetches the slashing spans record for a stash account, initializing it if necessary.
fn fetch_spans<'a, T: Config + 'a>(
    stash: &'a T::AccountId,
    window_start: EraIndex,
    paid_out: &'a mut BalanceOf<T>,
    slash_of: &'a mut BalanceOf<T>,
    reward_proportion: Perbill,
) -> InspectingSpans<'a, T> {
    let spans = <Module<T> as Store>::SlashingSpans::get(stash).unwrap_or_else(|| {
        let spans = SlashingSpans::new(window_start);
        <Module<T> as Store>::SlashingSpans::insert(stash, &spans);
        spans
    });

    InspectingSpans {
        dirty: false,
        window_start,
        stash,
        spans,
        slash_of,
        paid_out,
        reward_proportion,
        _marker: sp_std::marker::PhantomData,
    }
}

impl<'a, T: 'a + Config> InspectingSpans<'a, T> {
    fn span_index(&self) -> SpanIndex {
        self.spans.span_index
    }

    fn end_span(&mut self, now: EraIndex) {
        self.dirty = self.spans.end_span(now) || self.dirty;
    }

    // add some value to the slash of the staker.
    // invariant: the staker is being slashed for non-zero value here
    // although `amount` may be zero, as it is only a difference.
    fn add_slash(&mut self, amount: BalanceOf<T>, slash_era: EraIndex) {
        *self.slash_of += amount;
        self.spans.last_nonzero_slash = sp_std::cmp::max(self.spans.last_nonzero_slash, slash_era);
    }

    // find the span index of the given era, if covered.
    fn era_span(&self, era: EraIndex) -> Option<SlashingSpan> {
        self.spans.iter().find(|span| span.contains_era(era))
    }

    // compares the slash in an era to the overall current span slash.
    // if it's higher, applies the difference of the slashes and then updates the span on disk.
    //
    // returns the span index of the era where the slash occurred, if any.
    fn compare_and_update_span_slash(&mut self, slash_era: EraIndex, slash: BalanceOf<T>) -> Option<SpanIndex> {
        let target_span = self.era_span(slash_era)?;
        let span_slash_key = (self.stash.clone(), target_span.index);
        let mut span_record = <Module<T> as Store>::SpanSlash::get(&span_slash_key);
        let mut changed = false;

        let reward = if span_record.slashed < slash {
            // new maximum span slash. apply the difference.
            let difference = slash - span_record.slashed;
            span_record.slashed = slash;

            // compute reward.
            let reward = REWARD_F1 * (self.reward_proportion * slash).saturating_sub(span_record.paid_out);

            self.add_slash(difference, slash_era);
            changed = true;

            reward
        } else if span_record.slashed == slash {
            // compute reward. no slash difference to apply.
            REWARD_F1 * (self.reward_proportion * slash).saturating_sub(span_record.paid_out)
        } else {
            Zero::zero()
        };

        if !reward.is_zero() {
            changed = true;
            span_record.paid_out += reward;
            *self.paid_out += reward;
        }

        if changed {
            self.dirty = true;
            <Module<T> as Store>::SpanSlash::insert(&span_slash_key, &span_record);
        }

        Some(target_span.index)
    }
}

impl<'a, T: 'a + Config> Drop for InspectingSpans<'a, T> {
    fn drop(&mut self) {
        // only update on disk if we slashed this account.
        if !self.dirty {
            return;
        }

        if let Some((start, end)) = self.spans.prune(self.window_start) {
            for span_index in start..end {
                <Module<T> as Store>::SpanSlash::remove(&(self.stash.clone(), span_index));
            }
        }

        <Module<T> as Store>::SlashingSpans::insert(self.stash, &self.spans);
    }
}

/// Clear slashing metadata for an obsolete era.
pub(crate) fn clear_era_metadata<T: Config>(obsolete_era: EraIndex) {
    <Module<T> as Store>::ValidatorSlashInEra::remove_prefix(&obsolete_era);
    <Module<T> as Store>::NominatorSlashInEra::remove_prefix(&obsolete_era);
}

/// Clear slashing metadata for a dead account.
pub(crate) fn clear_stash_metadata<T: Config>(stash: &T::AccountId, num_slashing_spans: u32) -> DispatchResult {
    let spans = match <Module<T> as Store>::SlashingSpans::get(stash) {
        None => return Ok(()),
        Some(s) => s,
    };

    ensure!(num_slashing_spans as usize >= spans.iter().count(), Error::<T>::IncorrectSlashingSpans);

    <Module<T> as Store>::SlashingSpans::remove(stash);

    // kill slashing-span metadata for account.
    //
    // this can only happen while the account is staked _if_ they are completely slashed.
    // in that case, they may re-bond, but it would count again as span 0. Further ancient
    // slashes would slash into this new bond, since metadata has now been cleared.
    for span in spans.iter() {
        <Module<T> as Store>::SpanSlash::remove(&(stash.clone(), span.index));
    }

    Ok(())
}

// apply the slash to a stash account, deducting any missing funds from the reward
// payout, saturating at 0. this is mildly unfair but also an edge-case that
// can only occur when overlapping locked funds have been slashed.
pub fn do_slash<T: Config>(
    stash: &T::AccountId,
    value: BalanceOf<T>,
    reward_payout: &mut BalanceOf<T>,
    slashed_imbalance: &mut NegativeImbalanceOf<T>,
) {
    let controller = match <Module<T>>::bonded(stash) {
        None => return, // defensive: should always exist.
        Some(c) => c,
    };

    let mut ledger = match <Module<T>>::ledger(&controller) {
        Some(ledger) => ledger,
        None => return, // nothing to do.
    };

    let value = ledger.slash(value, T::Currency::minimum_balance());

    if !value.is_zero() {
        let (imbalance, missing) = T::Currency::slash(stash, value);
        slashed_imbalance.subsume(imbalance);

        if !missing.is_zero() {
            // deduct overslash from the reward payout
            *reward_payout = reward_payout.saturating_sub(missing);
        }

        <Module<T>>::update_ledger(&controller, &ledger);

        // trigger the event
        <Module<T>>::deposit_event(super::RawEvent::Slash(stash.clone(), value));
    }
}

/// Apply a previously-unapplied slash.
pub(crate) fn apply_slash<T: Config>(unapplied_slash: UnappliedSlash<T::AccountId, BalanceOf<T>>) {
    let mut slashed_imbalance = NegativeImbalanceOf::<T>::zero();
    let mut reward_payout = unapplied_slash.payout;

    do_slash::<T>(&unapplied_slash.validator, unapplied_slash.own, &mut reward_payout, &mut slashed_imbalance);

    for &(ref nominator, nominator_slash) in &unapplied_slash.others {
        do_slash::<T>(&nominator, nominator_slash, &mut reward_payout, &mut slashed_imbalance);
    }

    pay_reporters::<T>(reward_payout, slashed_imbalance, &unapplied_slash.reporters);
}

/// Apply a reward payout to some reporters, paying the rewards out of the slashed imbalance.
fn pay_reporters<T: Config>(
    reward_payout: BalanceOf<T>,
    slashed_imbalance: NegativeImbalanceOf<T>,
    reporters: &[T::AccountId],
) {
    if reward_payout.is_zero() || reporters.is_empty() {
        // nobody to pay out to or nothing to pay;
        // just treat the whole value as slashed.
        T::Slash::on_unbalanced(slashed_imbalance);
        return;
    }

    // take rewards out of the slashed imbalance.
    let reward_payout = reward_payout.min(slashed_imbalance.peek());
    let (mut reward_payout, mut value_slashed) = slashed_imbalance.split(reward_payout);

    let per_reporter = reward_payout.peek() / (reporters.len() as u32).into();
    for reporter in reporters {
        let (reporter_reward, rest) = reward_payout.split(per_reporter);
        reward_payout = rest;

        // this cancels out the reporter reward imbalance internally, leading
        // to no change in total issuance.
        T::Currency::resolve_creating(reporter, reporter_reward);
    }

    // the rest goes to the on-slash imbalance handler (e.g. treasury)
    value_slashed.subsume(reward_payout); // remainder of reward division remains.
    T::Slash::on_unbalanced(value_slashed);
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn span_contains_era() {
        // unbounded end
        let span = SlashingSpan { index: 0, start: 1000, length: None };
        assert!(!span.contains_era(0));
        assert!(!span.contains_era(999));

        assert!(span.contains_era(1000));
        assert!(span.contains_era(1001));
        assert!(span.contains_era(10000));

        // bounded end - non-inclusive range.
        let span = SlashingSpan { index: 0, start: 1000, length: Some(10) };
        assert!(!span.contains_era(0));
        assert!(!span.contains_era(999));

        assert!(span.contains_era(1000));
        assert!(span.contains_era(1001));
        assert!(span.contains_era(1009));
        assert!(!span.contains_era(1010));
        assert!(!span.contains_era(1011));
    }

    #[test]
    fn single_slashing_span() {
        let spans = SlashingSpans { span_index: 0, last_start: 1000, last_nonzero_slash: 0, prior: Vec::new() };

        assert_eq!(spans.iter().collect::<Vec<_>>(), vec![SlashingSpan { index: 0, start: 1000, length: None }],);
    }

    #[test]
    fn many_prior_spans() {
        let spans =
            SlashingSpans { span_index: 10, last_start: 1000, last_nonzero_slash: 0, prior: vec![10, 9, 8, 10] };

        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 10, start: 1000, length: None },
                SlashingSpan { index: 9, start: 990, length: Some(10) },
                SlashingSpan { index: 8, start: 981, length: Some(9) },
                SlashingSpan { index: 7, start: 973, length: Some(8) },
                SlashingSpan { index: 6, start: 963, length: Some(10) },
            ],
        )
    }

    #[test]
    fn pruning_spans() {
        let mut spans =
            SlashingSpans { span_index: 10, last_start: 1000, last_nonzero_slash: 0, prior: vec![10, 9, 8, 10] };

        assert_eq!(spans.prune(981), Some((6, 8)));
        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 10, start: 1000, length: None },
                SlashingSpan { index: 9, start: 990, length: Some(10) },
                SlashingSpan { index: 8, start: 981, length: Some(9) },
            ],
        );

        assert_eq!(spans.prune(982), None);
        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 10, start: 1000, length: None },
                SlashingSpan { index: 9, start: 990, length: Some(10) },
                SlashingSpan { index: 8, start: 981, length: Some(9) },
            ],
        );

        assert_eq!(spans.prune(989), None);
        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 10, start: 1000, length: None },
                SlashingSpan { index: 9, start: 990, length: Some(10) },
                SlashingSpan { index: 8, start: 981, length: Some(9) },
            ],
        );

        assert_eq!(spans.prune(1000), Some((8, 10)));
        assert_eq!(spans.iter().collect::<Vec<_>>(), vec![SlashingSpan { index: 10, start: 1000, length: None },],);

        assert_eq!(spans.prune(2000), None);
        assert_eq!(spans.iter().collect::<Vec<_>>(), vec![SlashingSpan { index: 10, start: 2000, length: None },],);

        // now all in one shot.
        let mut spans =
            SlashingSpans { span_index: 10, last_start: 1000, last_nonzero_slash: 0, prior: vec![10, 9, 8, 10] };
        assert_eq!(spans.prune(2000), Some((6, 10)));
        assert_eq!(spans.iter().collect::<Vec<_>>(), vec![SlashingSpan { index: 10, start: 2000, length: None },],);
    }

    #[test]
    fn ending_span() {
        let mut spans = SlashingSpans { span_index: 1, last_start: 10, last_nonzero_slash: 0, prior: Vec::new() };

        assert!(spans.end_span(10));

        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 2, start: 11, length: None },
                SlashingSpan { index: 1, start: 10, length: Some(1) },
            ],
        );

        assert!(spans.end_span(15));
        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 3, start: 16, length: None },
                SlashingSpan { index: 2, start: 11, length: Some(5) },
                SlashingSpan { index: 1, start: 10, length: Some(1) },
            ],
        );

        // does nothing if not a valid end.
        assert!(!spans.end_span(15));
        assert_eq!(
            spans.iter().collect::<Vec<_>>(),
            vec![
                SlashingSpan { index: 3, start: 16, length: None },
                SlashingSpan { index: 2, start: 11, length: Some(5) },
                SlashingSpan { index: 1, start: 10, length: Some(1) },
            ],
        );
    }
}
